1. Field of the Invention
The present invention is broadly applicable to the processing of carbonaceous feed material to effect desirable chemical and physical changes, usually through elevated temperatures and increased pressures. More specifically, this invention relates to the drying and/or gasification of solid fossil fuels and solid biological wastes by thermal restructuring or pyrolization to produce usable solids as well as gaseous and/or liquid by-products. Additionally, this invention relates to contaminated soil sterilization through the oxidation of organic toxins, and to the devolitilization and oxidation of toxins absorbed on spent beds of activated carbon or other materials, including inorganic adsorbents.
2. Description of the Prior Art
The fuel shortages of the mid-1970's and the projection of future shortages of such conventional energy sources as gas and oil have given rise to investigations into alternative energy sources, including solid fossil fuels, such as oil shales, brown coals, lignites, subbituminous coals and the like, and solid biological wastes, such as wood chips, saw dust, peat, sewage sludge and other carbonaceous sludges, bagasse, hulls, straw, cornstalks and the like.
Although this present invention applies to both solid fossil fuels and solid biological wastes, most of the relevant discussions in the prior art deals with coal conversion. For example, a summary of coal conversion is provided in "Coal Conversion Technology: Opportunities and Challenges", Energy, Volume II, Number 11/12, pages 1239-1247, 1986, by T. Y. Yan. In general, coal conversion processes fall into four groups: desulfurization, liquification, carbonization, and gasification. Desulfurization, which for most coals is only marginally achieved by this invention, is usually accomplished by chemical leaching. Liquification has been accomplished by mixing crushed coal with oil (often process derived) into a slurry and then reacting it with hydrogen under increased temperatures and pressures. Carbonization, or coal drying, which has traditionally been accomplished in the absence of air or other oxidants, involves a boiling away of the moisture contained within the coal, a thermal restructuring of the coal into a char, and the generation of gaseous and liquid by-products. In gasification, coal is reacted with oxygen and/or steam at increased temperatures and elevated pressures to form gaseous mixtures of hydrogen, carbon monoxide, and carbon dioxide.
Typical of current technology in coal drying are several of the prior art patents of Koppelman, including U.S. Pat. No. 4,052,168 and its two continuations-in-part, U.S. Pat. Nos. 4,127,391 and 4,129,420. The parent U.S. Pat. No. 4,052,168 describes a process for upgrading lignite-type coal by a batch autoclave treatment at elevated temperatures and increased pressure to effect a thermal restructuring of the coal and to generate a gaseous byproduct. The first continuation-in-part, U.S. Pat. No. 4,127,391, utilizes this autoclave treatment for agglomerating bituminous fines into useful low-sulfur fuel. The second continuation-in-part, U.S. Pat. No. 4,129,420, generates coke by heating cellulosic materials in the autoclave.
Other Koppelman patents that are typical of current technology in gasification include: U.S. Pat. No. 4,069,107, and its continuation-in-part U.S. Pat. No. 4,106,999. These two patents describe a process of continuous pyrolyzing particulated carbonaceous feed material in which preheated pebbles are mixed with the carbonaceous feed to form a downwardly moving columnar reaction mass. These two patents further describe the introduction of an oxidant into a cylindrical reaction chamber through pores in a concentric annular wall within the chamber.
Other prior art patents of interest include Murray, U.S. Pat. No. 4,126,519, and two other Koppelman patents, U.S. Pat. Nos. 4,477,257 and 4,626,258, all of which describe a countercurrent flow of the gaseous by-product, not the oxidant, to preheat the feed materials in their pyrolysis drying methods. U.S. Pat. No. 4,126,519 discloses a continuous drying method utilizing contiguous chambers for preheating and thermal reacting the feed material. U.S. Pat. No. 4,477,257 also shows a continuous drying method, in which the process is performed in pre-heating, pressurized dewatering and reaction stages with a separate chamber for each stage. The last of these patents U.S. Pat. No. 4,626,258, which also teaches a continuous process utilizing a multiple hearth reactor that contains a plurality of annular hearths.
The above-described prior art processes and other current methods of coal drying and gasification are characterized by slow and difficult transfer of heat from the heat source into the interior of the coal particles, thus resulting in slow and inefficient means for boiling moisture from the coil. One approach, called low temperature drying, has partially overcome this slow heat transfer problem by placing the coal in intimate contact with a drying gas, thereby creating a turbulent, fluidized bed. Disadvantages of this turbulent, fluidized bed approach include difficulties in controlling the fluidized beds, a narrow, confining range of operating conditions, and large equipment investments that afford only marginal economic returns. The most severe disadvantage, however, is that low temperature drying methods typically generate pyrophoric dried coal, which ignites spontaneously when exposed to air. Consequently, such pyrophoric dried coal product requires chemical treatment at added expense before it can be transported, handled, or stored.
An additional persistent problem with current coal drying and gasification methods is the production of waste products, such as tar and ash. These waste products create special handling and disposal problems, the solutions of which can significantly increase costs.